JP4840535B1 - Method for determining fastening force of laminated iron core and method for producing laminated iron core - Google Patents

Abstract

[PROBLEMS] To accurately determine a fastening force that generates appropriate sliding friction between laminations in order to reduce vibration by using a damping effect caused by sliding friction between laminations in a laminated iron core in which electromagnetic steel sheets are laminated. Provided are a method for determining a fastening force of a laminated core and a method for manufacturing a laminated core thereby.
A laminated iron core is constructed using electromagnetic steel plates, and is vibrated with a mechanical excitation force within a range of 1N to 1000N under different fastening forces, so that one or more vibration levels of the laminated iron core are obtained. The fastening force of the laminated core determines the fastening force that reduces noise after exciting the laminated core by determining the relationship between the vibration level and the fastening force based on the measurement result of the vibration level. Decision method.
[Selection] Figure 1

Description

  The present invention relates to a method for determining a fastening force of a laminated iron core in which electromagnetic steel sheets are laminated, and a method for manufacturing a laminated iron core thereby.

  Among electromagnetic steel sheets, directional silicon steel sheets containing Si and having a crystal orientation in the (110) [001] direction have excellent soft magnetic properties and are widely used as various iron core materials in the commercial frequency range. ing.

  However, such an electromagnetic steel sheet has magnetostriction due to excitation, and when an iron core (laminated iron core) manufactured by laminating electromagnetic steel sheets is excited, vibration (out-of-plane bending vibration) is generated due to magnetostriction of the electromagnetic steel sheet, resulting in noise. The source. Therefore, when a transformer (transformer) is manufactured using a laminated core, an electromagnetic steel sheet having a small magnetostriction is generally used as the core material in order to suppress the generation of noise. However, there are cases where the noise level of the transformer cannot meet the required specifications, even though an electromagnetic steel sheet having a small magnetostriction is used.

  Many of the causes are resonance phenomena of the natural vibration of the laminated iron core of the transformer and the magnetostrictive vibration of the magnetic steel sheet. That is, when the excitation power of the laminated core becomes an external force, forced vibration occurs, and when the excitation frequency matches the natural frequency of the laminated core, resonance occurs and the amplitude becomes large. For this reason, a method of designing a transformer by paying attention to the natural frequency of the laminated core has been studied.

  If the natural vibration of the laminated core resonates with the magnetostrictive vibration of the electrical steel sheet, the rigidity (spring constant) of the laminated core is changed so that the natural frequency of the laminated core changes to avoid the resonance. ) Will be changed. For example, the fixing condition of the laminated iron core is adjusted or changed, or the number of laminated electromagnetic steel sheets is changed.

  In addition, about the method of measuring the natural frequency of a laminated body core, it describes in patent document 1, for example. This is to know the natural frequency of the laminated core by changing the excitation frequency stepwise and measuring the noise generated by the laminated core.

JP 2008-82778 A

  However, a laminated iron core in which electromagnetic steel plates are laminated is a complex structure and has an infinite number of natural frequencies in a frequency band of 50 Hz to 20 kHz that can be a target of noise. In general, it is difficult to change only a specific natural frequency of a structure having a plurality of natural frequencies. When the rigidity is changed, the entire natural frequency is shifted. For this reason, in the method of changing the rigidity of the laminated core, even if the resonance between the natural vibration of one natural frequency component and the magnetostrictive vibration is avoided, the natural vibration of another natural frequency component is newly resonated with the magnetostrictive vibration. Very likely to do. In the first place, in the actual transformer design, since there are required specifications other than the natural vibration of the laminated core, it is not easy to change the rigidity of the laminated core.

  As a simple model, FIG. 8 schematically shows the relationship between frequency response characteristics (vertical axis: gain, horizontal axis: frequency) in the case of a one-degree-of-freedom vibration system and each parameter in the one-mass system model. In general, in order to avoid resonance, the rigidity UP (increase the spring constant k) as shown in FIG. 8A or the rigidity DOWN (increase the spring constant k) as shown in FIG. 8B. The following measures are taken. However, as described above, in the case of a laminated iron core, the method of avoiding resonance at the operating frequency by changing the rigidity is actually difficult.

  Therefore, as a countermeasure against vibration in the laminated iron core, the inventor considered a measure of attenuation UP (increase the attenuation coefficient c) as shown in FIG. In this method, the natural frequency does not change, but the vibration amplitude can be reduced. Furthermore, the inventor has found that the vibration amplitude can be significantly reduced by increasing the damping coefficient by using sliding friction between the layers (between the electromagnetic steel sheets constituting the laminated core).

  The detailed mechanism is shown in FIG. Here, each graph on the left side of FIG. 9 shows an external force (magnetostriction) pattern (vertical axis: magnetostriction, horizontal axis: time) applied to a transformer composed of a laminated core, and each central graph shows a fastening force (described later). ) Shows the frequency response function of the transformer (vertical axis: response amplitude (gain), horizontal axis: frequency). Further, each graph on the right side of FIG. 9 shows a pattern of transformer vibration (out-of-plane bending vibration) when the external force is applied to a transformer having the frequency response function (vertical axis: amplitude of iron core vibration, horizontal axis: Time).

  In the case of a laminated iron core, the shape cannot be fixed only by laminating electromagnetic steel sheets, and therefore, it is fixed by fastening means (clamping with a steel material, etc.). A compression force acts in the stacking direction when fixing by the fastening means, which is called a fastening force.

  As shown in FIG. 9A, when the fastening force is weak, the friction between the stacks is small, so that the magnetic steel sheets move in a direction that is shifted from each other due to the action of the external force, and the rigidity of the stack as a whole decreases. However, in this case, only the natural frequency shifts, and the vibration amplitude itself does not change greatly. In addition, since a plurality of natural frequencies shift, another resonance that has not been a problem may occur due to the low rigidity.

  On the contrary, as shown in FIG. 9C, when the fastening force is too large, the friction between the layers increases, and the laminate behaves like a solid rigid body, so that the rigidity as the laminate increases. . However, in this case as well, the natural frequency is merely shifted, and the vibration amplitude itself does not change significantly. As in the case of low rigidity, another resonance that has not been a problem may occur.

  On the other hand, when an appropriate fastening force is selected as shown in FIG. 9B, moderate sliding friction occurs between the layers. Since the sliding friction has a damping action, the damping element of the laminated structure is reinforced, and the vibration amplitude is greatly reduced even if the natural frequency does not change.

  With the mechanism as described above, it is possible to manufacture a laminated body core with low vibration by determining a fastening force that generates appropriate sliding friction between the laminated layers. And it becomes possible to manufacture a low noise transformer by using the low-vibration laminated iron core. However, it is difficult to accurately determine an appropriate fastening force in an actual machine or a conventional experimental facility.

  The present invention has been made in view of the circumstances as described above, and in a laminated core in which electromagnetic steel sheets are laminated, in order to reduce vibration by using a damping effect due to sliding friction between the laminations, It is an object of the present invention to provide a method for determining a fastening force of a laminated core that can accurately determine a fastening force that generates an appropriate sliding friction, and a method for manufacturing a laminated core using the same.

  In order to solve the above problems, the present invention has the following features.

  [1] A laminated iron core is constructed using electromagnetic steel plates, and is vibrated with a mechanical excitation force within a range of 1N to 1000N under different fastening forces, so that one or more vibration levels of the laminated iron core can be obtained. Laminate core characterized by determining the fastening force to reduce noise after exciting the laminate core by measuring and obtaining the relationship between vibration level and fastening force based on the measurement result of the vibration level Fastening force determination method.

  The vibration level is an index representing the magnitude of vibration, and is obtained by measuring at least any one of displacement, velocity, and acceleration (including calculation processing based on the measurement result). The measurement method is exemplified below, but is not limited thereto, and a known vibration analysis method can be applied.

  [2] As the vibration level, an amplitude RMS value or maximum amplitude value of a time-series waveform at a measurement location is calculated, and a representative value or an average value of the amplitude RMS value or maximum amplitude value is used. 1] The fastening force determination method of the laminated body core described in 1].

  [3] As the vibration level, frequency analysis is performed on a time-series waveform at a measurement location, a sum of frequency components N times the excitation frequency is calculated, and the value or average value of the representative point is used. The method for determining the fastening force of the laminated core according to [1].

  [4] As the vibration level, frequency analysis is performed on the time-series waveform at the measurement location, the sum of frequency components N times the excitation frequency is calculated, and the value or average value of the representative point is set as Wout. The laminated core according to [1], wherein the vibration force is subjected to frequency analysis, the sum of frequency components N times the excitation frequency is calculated, and the ratio Wout / Fin is used as Fin. Fastening force determination method.

  [5] Determine the fastening force using the method for determining the fastening force of the laminate core according to any one of [1] to [4], and manufacture the laminate core by fastening with the determined fastening force. A method for producing a laminated core, characterized by comprising:

  In the present invention, for a laminated core in which electromagnetic steel sheets are laminated, a fastening force that generates appropriate sliding friction between the laminations is obtained in order to achieve low vibration by using a damping effect caused by sliding friction between the laminations. Can be determined. Thereby, it becomes possible to manufacture a laminated iron core with low vibration. And it becomes possible to manufacture a low noise transformer by using the low-vibration laminated iron core.

  In addition, since the sliding friction phenomenon between the laminates in the laminate core can be confirmed by a partial model or a small model, an appropriate fastening force can be determined at a low cost without manufacturing a large-scale model.

  Conventionally, as shown in FIG. 10A, after manufacturing a laminated core and a transformer, when the vibration and noise are large, the fastening force is adjusted by trial and error. On the other hand, in this invention, as shown in FIG.10 (b), the suitable fastening force according to a use condition is calculated | required beforehand, and a laminated body core and a transformer can be manufactured with the appropriate fastening force. As a result, vibration and noise are reduced, and adjustment of the fastening force is unnecessary.

  Further, although it has been difficult to obtain the correlation between the fastening force and the vibration by the conventional method, in the present invention, a detailed and accurate correlation can be easily obtained. For this reason, in this invention, the presence or absence of the noise improvement room from the present fastening force and the potential improvement range can be grasped easily, and it can contribute to the design of the transformer.

It is a figure which shows Embodiment 1 of this invention. It is a figure which shows Embodiment 2 of this invention. It is a figure which shows Embodiment 3 of this invention. It is a figure which shows the laminated body core in Examples 1-3 of this invention. It is a figure which shows evaluation and determination of the fastening force in Example 1 of this invention. It is a figure which shows evaluation and determination of the fastening force in Example 2 of this invention. It is a figure which shows evaluation and determination of the fastening force in Example 3 of this invention. Here, (a) shows the result of evaluating the vibration level by acceleration, (b) shows the result of evaluating the vibration level by speed, and (c) shows the result of evaluating the vibration level by displacement. In order to explain the basic concept of the present invention, it is a diagram showing the influence of stiffness and damping coefficient on frequency response characteristics in a one-degree-of-freedom vibration system. It is a figure which shows the basic mechanism of this invention. It is a figure which shows the difference in the fastening force determination procedure with the conventional method, in order to demonstrate the basic concept of this invention.

  In the present invention, in a laminated core in which electromagnetic steel sheets are laminated, an appropriate fastening force that generates appropriate sliding friction between the laminations is used in order to reduce vibration by using a damping effect caused by sliding friction between the laminations. I try to decide.

  And the laminated iron core is manufactured by fastening the laminated electrical steel sheets with the fastening force determined as described above.

  Embodiments of the present invention will be described with reference to the drawings.

[Embodiment 1]
As shown in FIG. 1, a laminated iron core (preferably a small model or a partial model) 11 capable of adjusting a fastening force by a spring 13 through a bakelite 12 is prepared. Then, the waveform generated by the signal generator 21 is vibrated with a predetermined mechanical vibration force by the vibration exciter 22. Here, the predetermined mechanical excitation force is determined by vibration conditions or the like that may cause a problem in the actual use of the laminate core 11 to be measured, and is generally from a range of 1N to 1000N. Selected. The excitation force is measured by the load cell 23. The displacement, speed, or acceleration is measured by the vibration sensor 24 (when no vibration is applied, the time is zero), and a time series waveform is obtained. Analyzing the obtained time-series waveform, and obtaining the RMS value (root mean square) or the maximum value in amplitude as the representative value or average value of vibration (multiple measurement points or arithmetic average value of multiple measurements) (This is the vibration level.)

  Then, if the fastening force is changed and the vibration representative value (RMS value or maximum amplitude value) is sequentially measured, the relationship between the vibration level and the fastening force becomes clear. The same applies to the average value of vibration. Thus, an appropriate fastening force is determined according to the use situation of the laminated core.

[Embodiment 2]
As shown in FIG. 2, using a laminated iron core (small model or partial model) 11 whose fastening force can be adjusted by a spring 13 through a bakelite 12, a vibrator 22 with a sine wave fHz generated by a signal generator 21. To excite with a predetermined mechanical excitation force. Here, the predetermined mechanical excitation force is determined by vibration conditions or the like that may cause problems in the actual use of the laminate core (small model or partial model) 11 to be measured. Is a mechanical excitation force selected from the range of 1N to 1000N. The excitation force is measured by the load cell 23.

  Then, the displacement, velocity, or acceleration is measured by the vibration sensor 24 to obtain a time series waveform (when no vibration is applied, the zero point is set). The obtained time-series waveform is frequency-analyzed by the frequency analyzer 25, and the sum Wout of N-times components (N = 1, 2, 3,...) Of fHz out of the frequency components is obtained as a representative value of vibration (this) Is the vibration level). In addition, what is necessary is just to take an average, when calculating several points or measuring several times.

  Then, by changing the fastening force and sequentially measuring the representative value (Wout) of the vibration, the relationship between the vibration level and the fastening force becomes clear. Thus, an appropriate fastening force is determined according to the use situation of the laminated core.

  Compared with the method of the first embodiment, the method of the second embodiment can eliminate the amplitude level variation that occurs depending on the selected excitation force.

[Embodiment 3]
As shown in FIG. 3, using a laminated iron core (small model or partial model) 11 whose fastening force can be adjusted by a spring 13 via a bakelite 12, a vibrator 22 with a sine wave fHz generated by a signal generator 21. As in the second embodiment, vibration is performed with a predetermined mechanical vibration force.

  Then, the displacement, velocity, or acceleration is measured by the vibration sensor 24 to obtain a time series waveform (when no vibration is applied, the zero point is set). The obtained time series waveform is frequency-analyzed by the frequency analyzer 25 to obtain a total sum Wout of N-fold components (N = 1, 2, 3,...) Of fHz among the frequency components. In addition, what is necessary is just to take an average, when calculating several points or measuring several times. Simultaneously with calculating Wout, the excitation force is measured by the load cell 23 to obtain a time-series waveform of the excitation force. The obtained time-series waveform is frequency-analyzed by the frequency analyzer 26 to obtain the sum Fin of N frequency components (N = 1, 2, 3,...) Of fHz out of the frequency components. Then, Wout / Fin is set as a representative value (vibration level) of vibration.

  Then, if the fastening force is changed and the representative value (Wout / Fin) of vibration is sequentially measured, the relationship between the vibration level and the fastening force becomes clear. Thus, an appropriate fastening force is determined according to the use situation of the laminated core.

  The method according to the third embodiment has the same advantages as the method according to the second embodiment, and further, compared to the method according to the second embodiment, the influence on the vibration level due to fluctuations (fluctuations and changes with time) of the vibration force of the vibrator 22. Can be offset.

  Example 1 of the present invention will be described. In Example 1, an appropriate fastening force was obtained according to Embodiment 1 of the present invention described above.

  FIG. 4 shows a small laminated core 11 used in the first embodiment. As shown in the figure, this laminated core has a total of 5 pieces, each of which constitutes four sides, and a piece which constitutes the central portion, and is laminated in a step lap stacking manner. Here, L1 = 500 mm, L2 = 500 mm, L3 = 100 mm, L4 = 100 mm, and 70 electromagnetic steel sheets having a thickness of 0.2 mm are laminated. As the electrical steel sheet, a general grain-oriented electrical steel sheet is used. In addition, content of Si is 2.0-4.5 mass%.

  The excitation position was the midpoint of the side on the L1 side, and the measurement position of the vibration level was selected at three points at equal intervals on each side and the central piece, for a total of 15 points.

  FIG. 5 is a graph of the vibration average value when the fastening force is changed. As the vibration average value, the RMS value of the acceleration measured by the vibration sensor with an excitation force of 30 N (frequency sine wave of 100 Hz) was the 15-point average value.

As shown in FIG. 5, since when the fastening force is 6N / cm ² is low vibration, proper fastening force was determined to 6N / cm ^2.

  In addition, the maximum value of vibration is calculated | required instead of RMS value as an average vibration value at the time of changing fastening force, and it can also employ | adopt the fastening force with which the value becomes the smallest.

  Example 2 of the present invention will be described. In Example 2, an appropriate fastening force was obtained according to the second embodiment.

  Also in Example 2, the laminated iron core 11 having the same dimensions as in Example 1 is used, and the general steel sheet is also used as the electromagnetic steel sheet. The measurement position of the vibration position, vibration frequency and vibration level was also the same as in Example 1. In addition, content of Si is 2.0-4.5 mass%.

  FIG. 6 is a graph of representative vibration values when the fastening force is changed. As the vibration representative value, the sum Wout of N-fold components of fHz out of the frequency components in the time-series waveform of acceleration obtained by measuring with a vibration sensor with an excitation force of 20N was used. For Wout, an averaged value after calculation from 15 acceleration frequency components was adopted.

As shown in FIG. 6, because if the fastening force is 6N / cm ² is low vibration, proper fastening force was determined to 6N / cm ^2. Here, when the excitation force was changed to 30N and 40N and the same measurement was performed, the graph shifted up and down, but the case where the fastening force was 6 N / cm ² was low as in the case shown in FIG. It was a vibration.

  Example 3 of the present invention will be described. In Example 3, an appropriate fastening force was obtained by the above-described Embodiment 3.

  In Example 3, the laminated iron core 11 having the same dimensions as those in Examples 1 and 2 was used. However, an electromagnetic steel sheet having a lower iron loss than that in Examples 1 and 2 was used. In addition, content of Si is 2.0-4.5 mass%. The measurement position of the vibration position, vibration frequency and vibration level was the same as in Examples 1 and 2.

FIG. 7A is a graph of vibration representative values when the fastening force is changed. Wout / Fin ((m / s ² ) / N) obtained by frequency analysis of 15 points of acceleration (m / s ² ) and excitation force (N) with 20 N excitation force was used as the vibration representative value. . For Wout, an averaged value after calculation from 15 acceleration frequency components was adopted.

As shown to Fig.7 (a), since the case where fastening force is 8 N / cm < ² > is low vibration, appropriate fastening force was determined to be 8 N / cm < ² >. Here, when the excitation force was changed to 30 N and 40 N and the same measurement was performed, the graph shifted up and down, but the fastening force was 8 N / cm ² as in the case shown in FIG. The case was low vibration.

Similarly, FIG. 7B and FIG. 7C show Wout / Fin (((m / s) / N) or (obtained by frequency analysis of velocity (m / s) and displacement (m)). m / N)) is a representative vibration value. For Wout, an averaged value after calculating from the velocity or displacement frequency components at 15 points was adopted. Even when evaluated by speed and displacement, as shown in FIGS. 7 (b) and 7 (c), when the fastening force is 8 N / cm ² , the vibration is low, so the proper fastening force is 8 N / cm ² . It has been determined.

  Hereinafter, matters common to the present invention including the above embodiments will be supplemented.

  A typical electrical steel sheet used for the laminated iron core is characterized by containing Si: 2.0 to 4.5% by mass. However, since the present invention is hardly affected by the specifications of the electrical steel sheet (including the coating specifications), it is not limited to this composition, and the type (for example, directional electrical steel sheet, non-oriented electrical steel sheet). ) Is not limited. For example, if a required characteristic for an iron core is satisfied, a general cold-rolled steel sheet can be used as an electromagnetic steel sheet. The size and shape of the electromagnetic steel sheet, the size and shape of the laminated core, and the lamination method (the number of laminated sheets and how to combine the electromagnetic steel sheets) do not particularly affect the application of the present invention.

  The excitation frequency is preferably fixed to a predetermined value. As the predetermined value, the use frequency of the target laminated core is preferable, and when the use frequency has a width, it is preferable to select and use at least one representative frequency. For example, it is conceivable to fix at least one of commercial frequencies of 50 Hz and 60 Hz. However, it is not limited to a use frequency, For example, you may select the unified frequency for the comparison with another laminated body core. As long as the frequency is realizable (for example, 50 Hz to 20 kHz as described above), the absolute value of the frequency is not particularly limited. A sinusoidal wave is suitable for the excitation waveform, but it does not exclude application of other periodic waveforms such as a triangular wave and a rectangular wave.

  The vibration position and vibration level measurement position on the laminated core can be arbitrarily set, and in principle, the result can be obtained at any position.

When determining the fastening force to reduce noise after exciting (magnetizing) the laminated core from the relationship between the obtained vibration level and the fastening force (that is, when energizing the iron core), for example, the vibration level is the lowest. The fastening force to be obtained may be selected from the measurement points. However, as can be seen from FIGS. 5 to 7, in the region where the vibration reduction effect due to sliding friction appears, the change in the vibration level becomes concave with respect to the change in the fastening force, and within this region (for example, 5 to 5 in FIGS. 7N / cm ² or so, in FIG. 7 (a) ~ (c) the 6~8N / cm ² approximately in the region), it is considered that the noise reduction effect was found in the present invention is sufficiently expressed. Therefore, from the relationship between the obtained vibration level and the fastening force, regardless of the minimum value of the vibration level, the fastening force region where the vibration / noise reduction effect due to sliding friction is expressed is identified, and the relevant region is considered in consideration of various requirements. From the above, the fastening force can be selected as appropriate.

Examples of the range in which the fastening force is changed include a range that is possible for the target laminated core, a normal range, and a range that is appropriately extracted from these ranges. The range in which the fastening force is changed and the increment of the change can be specified to the extent that there is a fastening force region where the vibration / noise reduction effect due to sliding friction appears as described above from the relationship between the obtained vibration level and the fastening force. Just choose. Generally, it is preferable to select appropriately from the range of about 5 to ²⁰ N / cm ² .

  For example, two or more of displacement, velocity, and acceleration may be employed as the vibration level. If the relationship between the vibration level and the fastening force varies depending on the selection of the vibration level, the most appropriate relationship may be determined based on the result of actually manufacturing the laminated core.

  Note that the influence of the movement of the resonance peak due to the change of the fastening force (for example, the increase in the vibration level due to the peak position approaching the measurement frequency) was not clearly observed as far as the inventors investigated. In other words, it is considered that the vibration level damping effect by the sliding friction of the present invention is more dominant than the influence of the movement of the resonance peak position that is originally complicatedly overlapped. It was not.

  Needless to say, the present invention is not limited to the above-described embodiments and examples. For example, the mechanism for applying the fastening force is not limited to the springs shown in FIGS.

  According to the present invention, there is provided a relatively simple method with high accuracy and high versatility that reduces vibration and noise of a laminated core.

11 Laminated iron core (model)
12 Bakelite 13 Spring 21 Signal generator 22 Exciter 23 Load cell 24 Vibration sensor 25 Frequency analyzer 26 Frequency analyzer k Spring coefficient in one-degree-of-freedom vibration system (based on one-mass system model)
c Damping coefficient in one-degree-of-freedom vibration system (by one-mass system model)
m Mass of mass point in 1-DOF vibration system (by 1-mass system model)

Claims (5)

  A laminated iron core is constructed using electromagnetic steel sheets, and under different fastening forces, it is vibrated with a mechanical excitation force within the range of 1N to 1000N, and the vibration level at one or more locations of the laminated iron core is measured. Based on the measurement result of the vibration level, the fastening force of the laminated core is determined by determining the fastening force that reduces noise after exciting the laminated core by obtaining the relationship between the vibration level and the fastening force. Decision method.   2. The amplitude RMS value or maximum amplitude value of a time-series waveform at a measurement location is calculated as the vibration level, and a representative value or average value of the amplitude RMS value or maximum amplitude value is used. Method for determining the fastening force of laminated iron cores.   2. The frequency level of a time-series waveform at a measurement location is calculated as the vibration level, the sum of frequency components N times the excitation frequency is calculated, and the value or average value of the representative point is used. The fastening force determination method of the laminated body core described in 1.   As the vibration level, frequency analysis is performed on the time-series waveform of the measurement location, the sum of frequency components N times the excitation frequency is calculated, the value of the representative point or the average value is set as Wout, and the excitation force is 2. A method of determining a fastening force of a laminated core according to claim 1, wherein frequency analysis is performed, a sum of frequency components N times the excitation frequency is calculated, and the ratio Wout / Fin is used as Fin. .   A laminated body characterized in that a fastening force is determined using the fastening force determination method for a laminated core according to any one of claims 1 to 4 and is fastened with the determined fastening force to produce a laminated core. Manufacturing method of iron core.